Metallocene catalyst systems are extensively used in a variety of olefin polymerisation systems, including the polymerisation of ethylene. Metallocene catalysts prepare polyethylene different from Ziegler-Natta and Phillips chromium-based catalysts in that they are generally single-site catalysts, thus suitable for polyethylene having a narrow molecular weight distribution, but also a uniform comonomer incorporation, not only on short polymer chains, but equally so on long polymer chains (a narrow comonomer distribution, preferably a comonomer distribution breadth index of greater than 50%). Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane activating agent, such as methylaluminoxane (MAO). This resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerisation media. These homogenous catalyst systems have the disadvantage that when they are used under slurry polymerisation conditions, they produce polyethylene, which stick to the reactor walls during the polymerisation process (generally referred to as “fouling”) and/or polyethylene having small particle size and low bulk density which limit their commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. Typically, these procedures have involved the prepolymerisation of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a “particulate solid” or “support”). The porous carrier is usually a silica-containing support. However, although such supports reduce fouling, they also decrease the catalytic activity i.e. the yield of solid polymer that is obtained by employing a given quantity of catalyst in a given amount of time.
There is an ongoing search for metallocene catalysts and techniques for preparing such catalysts which give improved activity for the polymerisation of olefins. An improved activity means that less metallocene catalyst needs to be used to polymerise more ethylene, thereby reducing the costs considerably, since metallocenes are more expensive than Ziegler-Natta and chromium catalysts. As a result, the polyethylene would have a reduced catalytic residue, which also means less volatiles and better organoleptic properties. In particular, it has been proven more difficult to improve the activity of metallocene catalysts in the preparation of high density polyethylene i.e. above 0.945 g/cc. This is because for high density polyethylene, less comonomer is used. The activity of lower density polyethylene, below 0.945 g/cc, is generally higher since the presence of comonomer increases yield and activity.
Several attempts have been made to titanate silica supports for use in metallocene catalysed ethylene polymerisations. Jongsomjit et al. (Molecules 2005, 10, 672, Ind. Eng. Chem. Res. 2005, 44, 9059 and Catalysis Letters Vol. 100, Nos. 3-4, April 2005) discloses the titanation of silicas for zirconocene catalysed ethylene polymerisation, wherein the support is allegedly prepared according to Conway et al. (J. Chem. Soc., Faraday Trans. J, 1989, 85(1), 71-78), such that (without being bound to theory) the titania is mixed throughout the catalyst support. Without being bound to theory it is thought that the activity is limited, because the catalyst grains are not rendered fragile enough to burst during polymerisation and free up active sites. In addition, the interaction of the Ti with the actives sites is not optimized. Moreover, the interaction of the MAO with the TiOH and/or SiOH is different.
EP 0882 743 discloses a titanation procedure wherein the titanium compound is pumped as a liquid into the reaction zone where it vaporises to titanate a supported chromium-based catalyst. This procedure is strictly applicable to chromium catalysts (Philipp's type catalysts) i.e. only supported chromium catalysts are titanated in this way in order to obtain shorter polymer chains during polymerisation of olefins. There is no incentive to titanate a support (which does not contain any chromium) in the same way for use in metallocene catalysed olefin polymerisations with the hope of increasing the catalyst system's activity. Chromium catalysts are an entirely different class of catalyst from metallocenes, the latter being single-site and much more sensitive to poisons. They undergo such completely different reaction mechanisms that polyolefins prepared with chromium catalysts and metallocene catalysts have very different molecular structures, notably metallocenes provide polyolefins with narrower molecular weight distributions. In addition, chromium catalysts after being titanated require severe activation conditions, e.g. activation temperatures of at least 700° C., so that the titanium compounds ignite to yield at least partially TiO2. In any case, this system is limited to polyethylene prepared with a chromium-based (Phillips-type) catalyst and is not applicable to metallocene catalysed polyethylene.
Thus, as explained here above, recent developments have improved the marketability of metallocene-catalysed polymers. However, in such polymers, the catalytic residue is still high, particularly in high density polyethylene prepared with metallocene catalysts. The catalytic residue needs to be reduced in other to make the polyethylene suitable for membranes, medical appliances, food packaging etc. Thus, a high density polyethylene is needed with a reduced catalytic residue.
It is a further objective of the invention to provide a polyethylene with less volatiles and good organoleptic properties.